This invention relates to an electromagnetic flow meter little affected by noises, particularly electrochemical noises.
An electromagnetic flow meter known to date is, for example, the type of FIG. 1 of U.S. Pat. No. 4,206,641. This conventional electromagnetic flow meter comprises an electromagnetic flow meter detector (hereinafter referred to as "the detector") and an electromagnetic flow meter converter (hereinafter referred to as "the converter"). The detector consists of a pipe provided with a pair of electrodes and a pair of excitation coils set on both sides of the pipe. The converter is comprised of an amplifier, inversion type amplifier, integration circuit and control circuit.
The excitation coils are supplied with a square wave current which has the frequency of a commercial power source divided by an even number. As a result, the excitation coils generate a square wave magnetic field. When an electroconductive fluid, for example, city water or chemical, is conducted through the above-mentioned pipe, voltage is induced between the pair of electrodes according to Faraday's law, and the induced voltage is amplified to a prescribed level by the amplifier of the converter. The amplified voltage is then inverted.
The amplified voltage and inversion amplified voltage are alternately selected and converted into a signal of negative voltage. The signals of negative voltage are subjected to sampling at a prescribed time and delivered to the integration circuit. The integration circuit sends forth (outputs) a signal, obtained by the integration of the input signal, to the control circuit. The resultant pulse signal is smoothed to determine the flow rate of the subject fluid
The conventional square wave excitation type electromagnetic flow meter performing the abovementioned operation offers the undermentioned advantages:
(1) A loop constituted by the electrodes and in which the amplifier the and fluid is essentially free from the occurrence of rectangular noises and noises of the same phase, thereby ensuring the issue of a stable output signal.
(2) The sampling of the flow rate signal is carried out per period of the frequency of a commercial power source, thereby eliminating the noises which are generated by the induction of the commercial power source (high resistance to noises).
(3) An amplified signal and inversion amplified signal are alternately sent forth, thereby effectively eliminating noises resulting from the DC component of, for example, the offset voltage of the amplifier and of an ultra-low frequency.
(4) Since various noises are effectively eliminated, a highly stable output signal can be issued, even if the excitation current is reduced in quantity and the flow rate signal is of a low level. Consequently, the above-mentioned conventional electromagnetic flow meter can be operated on little power, the electromotive force generated per unit flow rate in the aforesaid electrodes being reduced to a level as low as one-fifth to one-tenth of what is observed in an electromagnetic flow meter excited by a commercial power source.
One of the noises accompanying the conventional electromagnetic flow meter is of the electrochemical type. This electrochemical noise arises with ultra-low frequency when electric charges of the electrodes are moved by the ions of the aforementioned fluids. This electrochemical noise raises or drops the voltage induced across the paired electrodes. The electrochemical noise has a level varying with the component of the fluids and the material and surface condition of the paired electrodes. Particularly with respect to a slurry fluid, the electrochemical noise of the ultra-low frequency is indicated as being at a high level.
When the level of the noise falls below the level allowed by the converter, the electrochemical noise is eliminated by the converter, thereby preventing the measurement of the subject electromagnetic flow meter from being reduced in precision. Conversely, when the electrochemical noise has a higher level than permitted by the converter, an amplifier is set to operate in the saturation region in the converter, giving rise to irregular functions and noticeably decreasing the measurement precision. For instance, in the above-mentioned conventional electromagnetic flow meter, the electrochemical noise is of a sufficiently high level to cause an input signal to the integration circuit to be of a positive voltage, causing the control circuit to fail to perform a normal function. For the resolution of this problem, the conventional practice is to supply a negative bias voltage to the integration circuit to expand the range of stable operation. However, the impression of a bias voltage to the integration circuit leads to a reduction of the extent to which a flow rate signal can vary within the operational range of the integration circuit and, moreover, the noises of the integration circuit exert a more prominent effect in the later stage of the integration circuit. Consequently, the drift of the offset voltage of the integration circuit is, for example, enlarged, thus imposing limitations on the extent to which the bias voltage can be impressed.
Hitherto, therefore, a limiter circuit has been provided in the detector to eliminate electrochemical noises. This limiter circuit comprises Zener diodes connected together with their polarities reversed with respect to each other, and resistors connected in parallel with the Zener diodes. However, though the conventional electrochemical noise-eliminating process involving the limiter circuit can indeed restrict the occurrence of electrochemical noises, at the same time it also restricts the issue of a flow rate signal. Therefore, the conventional process can not yet be regarded as fully adapted to attain the intended object.